My sincerest thanks. I have been digging a little on Nikon's website -- of the full frame cameras given there the D750 is the lowest grade to have EFCS, but the price is prohibitive. It would actually be significantly cheaper to buy a lesser camera and one of the made-to-order eight-hundred Euro adapter from LMScope.
I don't know where to begin with secondhand cameras -- is there a Nikon forum?
The matter must not be as impenetrable as I find it, there will be a solution. I suspect at least part of my problem is reasoning from theory before I've actually handled the equipment, but you have my sincerest thanks. Elementary photography is beating me
Incidentally, if I didn't live in such a remote place (two hundred miles north of Edinburgh, fifty north of Inverness) I'd inquire if someone could simply demonstrate coupling a camera in person.